1. Technical Field
The present disclosure relates to a wireless mobile device, such as a wearable device, and a transmission method.
2. Description of the Related Art
The use of smart devices typified by smartphones has rapidly spread in recent years. In particular, wearable devices are attracting attention as downsized, advanced smart devices. Watch-type terminals having a short-range wireless communication function and used in association with smart phones and wireless mobile terminals having a cellular communication function have already been commercialized.
Commercializing wearable devices requires downsizing antennas. However, downsizing a wearable device results in a size reduction of a circuit substrate included in the wearable device, an increase in the antenna current distribution density of the circuit substrate, and an increase in the specific absorption rate (SAR) value. Typically, it is difficult to reduce the SAR of a small terminal.
The SAR evaluation criterion and regulation value vary with the use state of the wireless mobile terminal. For watch-type terminals having a cellular communication function, conceivable use states thereof include a state in which the user is using a watch-type terminal with it mounted on his or her wrist, as well as a state in which the user is making a call with a terminal device brought close to his or her mouth or head. For this reason, there are two SAR evaluation criteria: a “wrist-worm” criterion assuming that the terminal is mounted on a wrist and a “mouth-worm” criterion assuming that the using is making a call with the terminal brought close to the mouth or head.
For example, the wrist-worm criterion requires that the SAR measured with the back of the terminal adhering to a phantom satisfy a regulation value of 4 W/kg averaged over 10 gram of tissue; the mouth-worm criterion requires that the SAR measured with the front of the terminal spaced from a phantom by 10 mm satisfy a regulation value of 1.6 W/kg averaged over 1 gram of tissue. That is, the mouth-worn regulation value is set to a more stringent value than that of the wrist-worn regulation value.
With regard to wireless mobile terminals having transmission functions of multiple wireless systems, there is also stipulated the SAR when simultaneously performing the transmission functions of the multiple wireless systems. For example, it is assumed that a watch-type terminal has a cellular communication function and a wireless local area network (LAN) communication function and that the user makes a voice call using the cellular communication function and simultaneously performs a data communication using the wireless LAN communication function.
In this case, the sum of the SAR when transmitting signals in the 2 GHz band using the cellular communication function and the SAR when transmitting signals in the 2.4 GHz band using the wireless LAN communication function must satisfy a regulation value. The SAR regulation value when simultaneously performing the transmission functions of multiple wireless systems is set to the same value as the regulation value when performing the transmission function of a single wireless system. As described above, the mouth-worn regulation value is 1.6 W/kg averaged over 1 gram of tissue. That is, the SAR characteristics when using multiple transmission functions are regulated more stringently than those when using a single transmission function.
Japanese Patent No. 5194171 discloses a method for improving the SAR characteristics when simultaneously performing the transmission functions of multiple wireless systems. The method includes determining whether a human body is present within a predetermined distance from a terminal and, if the human body is determined to be present within the predetermined distance from the terminal, reducing the transmission power (maximum allowable level). Japanese Unexamined Patent Application Publication No. 2013-48400 discloses a method of controlling the transmission power so that the SAR becomes equal to or smaller than a predetermined reference value.